1. Field of the Invention
The present invention relates to an electromagnetic drive-type die casting decompression valve, a drive method for such a valve and a die casting unit, and particularly to an improved structure for a decompression valve that can be used with a decompression die casting method.
2. Description of Related Art
In injection molding machines, such as die caster machines, casting is generally carried out using a low pressure die casting method or vacuum die casting method for carrying out injection molding by extracting gas inside a cavity being formed inside a mold at the time of injecting metallic material. This type of low-pressure die casting method is adopted in order to prevent variations in quality and defects in cast items due to contained gas. Specifically, if a metallic material in a molten or semi-molten state is injected and filled at high speed and low pressure into the inside of a cavity that is not in a low pressure or vacuum state, the metallic material becomes turbulent inside the cavity and gas is convoluted with the metallic material, as a result of which defects, such as blow holes, occur in the cast item.
In order to overcome this type of problem, techniques are known for suppressing gas contained in a cast item to prevent variations in quality and defects by casting a metallic material using a die casting unit employing a low-pressure die casting method.
For example, the specification of Japanese patent No. 1640217 discloses technology relating to a valve drive unit 5 used in die casting as shown in FIG. 8. Specifically, Japanese patent No. 1640217 discloses a valve drive unit 5 used in die casting, having a gas extraction valve 13 arranged in a gas extraction hole 12 leading from a cavity 11 inside a mold 10, where in a valve open state gas is guided to the outside of the mold 10 so as fill the inside of the cavity 11 with molten metal, and if a molten metal detection sensor 14 provided between the cavity 11 and the gas extraction valve 13 detects molten metal, the gas extraction valve 13 is closed. This valve drive unit 5 disclosed in Japanese patent No. 1640217 uses an air driven valve in the gas extraction valve 13, and implements an improved drive structure, to enable implementation of an opening and closing operation of the gas extraction valve in a short time.
Besides an air-driven decompression valve (gas extraction valve 13) adopted by the specification of Japanese patent application No. 1640217, there is also an electromagnetically driven decompression valve (refer, for example, to Japanese patent laid-open No. 2002-239704).
Closing of the decompression valve of the previously described die casting unit is preferably carried out as soon as possible after detection of molten metal by the molten metal detection sensor when filling the inside of the cavity with molten metal, from the point of view of preventing reduction of a degree of vacuum inside the cavity. The specification of Japanese patent application No. 1640217 and also the invention of Japanese patent laid-open No. 2002-239704 relate to improvement in opening and closing operation response of a decompression valve, and to improvement in quality of a cast item.
However, with the specification of Japanese patent application No. 1640217 and the invention disclosed in Japanese patent laid-open No. 2002-239704, in order to improve the opening and closing operation response of the decompression valve the valve is operated at high speed, which, means that there is a problem of the valve bouncing at the time of closing the decompression valve. As described previously, closing of the decompression valve is carried out in a state where molten metal has approached to directly in front of the decompression valve, which means that if the valve bounces there will be a problem of molten metal infiltrating to the inside of the decompression valve. In this case, it will be necessary to dismantle the entire decompression valve, requiring a long time to deal with the problem, and also incurring expensive repair costs.
There is also technology for preventing decompression valve bounce arising, as disclosed in the specification of Japanese patent application 1699815. Specifically, the invention disclosed in the specification of Japanese patent application 1699815 is fitted with an absorption plate moving in accordance with rising of the valve, at the time of a valve closing operation, thus absorbing valve movement energy using the absorption plate to prevent valve bounce. By adopting this type of structure, with the invention disclosed in the specification of Japanese patent application 1699815, shortening of valve operation time and closing of an exhaust vent are assured. However, changing a decompression valve to this type of structure increases the size of the decompression valve in line with installation of the absorption plate, and also increases manufacturing cost. Also, when the problem of molten metal infiltration arises, similarly to the invention disclosed in Japanese patent laid-open No. 2002-239704 and the specification of Japanese patent application No. 1640217, it will be necessary to dismantle the entire decompression valve, and since the structure is complicated, it will require time and repair expense to deal with problems arising in the related art.